1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and more particularly to a control system for an internal combustion engine having a plurality of cylinders, provided with a cylinder halting mechanism for halting operation of some of the cylinders.
2. Description of the Related Art
Japanese Patent Laid-Open No. Hei 8-105339 discloses a control system for an internal combustion engine provided with a cylinder halting mechanism. In this control system, partial-cylinder operation in which some of cylinders of the engine are halted, and all-cylinder operation in which all of the cylinders are operated, are switched according to an engine load, i.e., a throttle valve opening. Specifically, according to this control system, the partial-cylinder operation is performed when the throttle valve opening TPS is less than a switching throttle valve opening TPS1 which is set according to the engine rotational speed Ne.
According to the conventional control system described above, if the throttle valve opening TPS during the partial-cylinder operation exceeds the switching throttle valve opening TPS1 and then becomes lower than the switching throttle valve opening TPS1 immediately, the operation mode of the engine changes from the partial-cylinder operation to the all-cylinder operation and then changes back to the all-cylinder operation immediately. That is, the operation mode of the engine changes frequently in a short time period. It is well known in the art that, when the throttle valve opening TPS is compared with the switching throttle valve opening TPS1, and the operation mode of the engine is switched according to a result of the comparison, the comparison is performed with hysteresis. However, even by the comparison with hysteresis, frequent switching sometimes occurs, and there is a problem that a switching shock (a sudden change of the engine torque by the switching) occurs frequently, or sufficient improvement in the fuel efficiency cannot be obtained.